The disclosure relates generally to gas turbines engines, and in particular, to cooling of a reheat combustor in a gas turbine engine.
A conventional gas turbine engine includes a compressor for compressing air (sometime referred to as an oxidant as the air has oxidizing potential due to the presence of oxygen), which is mixed with fuel in a combustor and the mixture is combusted to generate a high pressure, high temperature gas stream, referred to as a post combustion gas. The post combustion gas is expanded in a turbine (high pressure turbine), which converts thermal energy from the post combustion gas to mechanical energy that rotates a turbine shaft.
Generally, during the process of combustion in the combustor, the oxygen content in the air is not fully consumed. As a result, the hot post combustion gas, exiting from the high pressure turbine, is associated with approximately 15% to approximately 18% by mass of oxygen and therefore has the potential of oxidizing more fuel. Some gas turbine engines, therefore, deploy a reheat combustor, where the post combustion gas is re-combusted after mixing with additional fuel. The re-combusted post combustion gas is expanded in another turbine section (low pressure turbine) to generate additional power. The deployment of the reheat combustor and the low pressure turbine therefore utilises the oxidizing potential of the post combustion gas, thereby increasing the efficiency of the engine.
The reheat combustors, however, during operation, possess a high demand for cooling air, which is generally provided by extracting a stream of air from the compressor. The extraction of air reduces the engine efficiency, as the stream of extracted air is unavailable for expansion in the high pressure turbine. The extraction of compressor air for cooling the reheat combustor therefore reduces the benefits of deploying the reheat combustor. Accordingly, a turndown or complete shutoff of the reheat combustion system may be desired. In a turndown situation, the engine may be requested to produce only a portion of its power, and may be turned down to as little as 20% of full load. A fixed flow area system (not modulated) would lead to unacceptable losses in power since the reheat cooling flow would remain at a fixed percentage of the total flow regardless of the firing conditions. In turn, this would severely restrict the range of firing temperatures, emissions compliance, complete fuel burning, and turn-down.
In addition, in a reheat combustor including an open centerline, hot combustion gasses are able to access an upstream intermediate pressure turbine (IPT) rear frame (diffuser). In a reheat combustor of this design, flame stabilization devices are typically cantilevered from the outer radius inward, which leads to cooling issues for the flame stabilization devices. Typically, cooling air will need to be injected through the flame stabilization devices, exiting into the hot combustion gases. This open centered design creates an imaginary flow boundary through symmetry, such that hot gases will be recirculated in a toroidal cell on the centerline. This flow pattern will circulate hot gas back to the IPT rear frame structure, causing an increased active cooling need for that structure and present hot gas issues circulating around the entire surface of the flame stabiliser. The unsteady nature of the recirculation zone will additionally present issues for the combustor's overall stability.
It is therefore desirable to have an alternate method to cool a reheat combustor without adversely affecting the engine efficiency.